There exists an apparatus for observing a specimen by using an electron beam which is known as a scanning electron microscope referred to hereafter simply as an SEM. In addition, as one of apparatuses for inspecting a semiconductor device, there is known a scanning electron microscope for length measurement referred to as a length measurement SEM. However, while the ordinary SEM and the length measurement SEM are suited for observation of a limited field of vision at a high magnification, they are unsuitable for finding the location of a defect on a wafer. This is because, in order to find the location of a defect on a wafer, it is necessary to search an extremely large area of the wafer or the entire surface of the wafer with a high degree of scrutiny. It takes a very long time to search such an extremely large area by using an ordinary or length measurement SEM because the current of the electron beam thereof is small, resulting in a slow scanning speed. As a result, if such SEMs are used in a process to fabricate a semiconductor device, the time it takes to complete the processing steps becomes very long, making the SEMs apparatuses of no practical use.
As an apparatus used for solving the problems described above, there is known an inspection apparatus using an electron beam for detecting a defect on a wafer by comparison of pictures. The apparatus is characterized in that: a large current electron beam is used; a specimen stage is continuously moved while the electron beam is being radiated to a specimen; a high acceleration voltage is used to accelerate the electron beam generated by an electron source; a retarding voltage is applied to the specimen to reduce the speed of the electron beam so as to prevent the specimen from being electrically-charged; and charged particles emanating from the specimen due to the radiation of the electron beam are detected after passing through an objective lens in a so-called TTL (through the lens) system.
As a result, the apparatus described above allows a mask or a pattern on a wafer serving as a specimen to be inspected for a defect with a higher degree of efficiency than the conventional SEM. It should be noted that this related technology is disclosed in documents such as Japanese Patent Laid-open No. Hei 5-258703.
With the TTL system whereby charged particles emanating from a specimen are detected after passing through an objective lens, the distance from the specimen to the objective lens can be shortened. As a result, the objective lens can be used at a short focal distance, allowing the amount of aberration of the electron beam to be reduced and, hence, a high-resolution picture to be obtained. On the other hand, the TTL system brings about unnegligible problems such as a hindrance to the improvement of the scanning speed and a big rotation change of the electron beam accompanying a variation in specimen height, causing a resulting picture to rotate as well.
FIG. 13 is a diagram showing a relation between the retarding voltage and the efficiency of detection of secondary electrons. Curve (2) shows a relation for the TTL system. As shown by curve (2), the TTL system has a problem that, as the retarding voltage is reduced, the efficiency of detection of secondary electrons also decreases to such a small value that the problem caused by a low detection efficiency can not be ignored anymore. Secondary electrons emanating from a specimen converge after passing through a magnetic field of an objective lens. The position of convergence in the axial direction changes when the retarding voltage is modified due to a variation in electron beam radiation energy. This phenomenon is the main reason why the efficiency of detection of secondary electrons decreases.